The present invention relates to the field of electronic circuits, and, more particularly, to converters, adapters, battery chargers and similar circuits. Specifically, the invention relates to a low consumption converter directly connectable to the mains or an AC power source.
Power supplies typically include a DC-DC converter coupled to an AC power source (e.g., the mains) through one or more stages. In a pulse width modulation (PWM) switching converter, a square wave drives the control terminal of a power switch and determines whether it is conductive or not conductive. The output voltage is increased by increasing the duration of the phase of conduction of the switch, and decreased by increasing the duration of the phase during which there is no conduction. Thus, the output voltage is controlled by varying the duty cycle of the driving square wave.
When the power switch is a MOS transistor, a non-negligible amount of power is spent to periodically charge the gate of the switching transistor. Power dissipation increases with an increase in the switching frequency and noticeably affects the overall efficiency of the converter. In particular, such a power dissipation lowers the efficiency of the converter, which is particularly true when the load is relatively small. For this reason, typical prior art devices provide for a lowered switching frequency when supplying a relatively small load.
In the following description reference will be made to a two stage converter, as illustrated in FIG. 1, because of its far greater diffusion than other types of converters. Yet, the following considerations are equally applicable to a converter with a number of stages greater than two.
As illustrated in FIG. 1, a two stage converter may include a rectifier coupled to an AC power source, a power factor correction pre-regulating circuit PFC supplied with the rectified voltage and producing a DC voltage of a certain nominal value. Further, a DC-DC converter is input with the nominal DC voltage and controls a load. The DC-DC converter may be based on a control scheme of any suitable kind (e.g., PWM, quasi resonant, resonant, etc.).
A well known solution to reduce energy consumption under relatively small or null load conditions includes reducing the switching frequency of the switches of the PFC stage or of the DC-DC stages, separately. Even if the energy savings that can be obtained thereby is not negligible, the energy consumption remains conspicuously large in the case of systems that remain in a stand-by state or supply a relatively small load for a long time.
Several converters which attempt to address this problem are known in the art. Such converters show enhanced performance in terms of energy savings because, under conditions of relatively small load, the PFC pre-regulating circuit is automatically set to a low consumption (quiescent) condition. On the contrary, when the load increases the PFC resumes its normal functioning. This provides for an increased output power requisite.
By turning off the PFC, the voltage on the bulk capacitor Co that couples the PFC to the converter drops from the level of the regulated voltage to the lower rectified and filtered network voltage. Considering the fact that the leakage current of the capacitor increases as the applied voltage increases approximately according to the following formula:
Ileakxe2x89xa70.02*C(xcexcF)*VR(V)+15 xcexcA,
the turning off of the PFC reduces capacitor losses. Moreover, lowering the voltage on the capacitor Co and the voltage supplied to the converter reduces the switching losses of the converter and the PFC.
U.S. Pat. No. 5,903,138 discloses a two-stage switching regulator that operates in one of four functioning modes, selected according to load conditions. A drawback of this regulator is the fact that it requires a relatively complicated logic circuit for switching from one functioning mode to another. Moreover, such a regulator may change its functioning mode even if the load remains constant, and it may cause the generation of electric noise at audible frequencies.
U.S. Pat. No. 5,726,871 discloses a power supply circuit for a video display capable of reducing power consumption. To this end, the functioning of its power factor correction is controlled by an external microcomputer depending on functioning conditions of the video display.
Furthermore, U.S. Pat. No. 5,960,207 discloses a power supply including a power factor correction and a controller that disables the power factor correction when the power supply is operating in a low power mode. Monitoring of the load is carried out in the secondary circuit of the power supply which is isolated from the primary circuit according to safety rules. Therefore, the control command produced by the controller for disabling the power factor correction must be transmitted to the primary circuit of the power supply using a device appropriate to keep such an isolation, such as an optoelectronic switching coupler.
In view of the foregoing background, it is therefore an object of the invention to provide a converter directly connectable to an AC power source (e.g., the mains) that allows a significant reduction of power dissipation by turning off its PFC when the load is smaller than a certain threshold, substantially avoiding the risk of generating electrical noise at audible frequencies.
It is another object of the invention to provide such a converter which has a relatively simple control circuit and that may be realized in an integrated form.
This and other objects, features, and advantages in accordance with the present invention are provided by a converter that is directly connectable to the mains that includes a rectifier stage for rectifying a network voltage and a power factor correction pre-regulating circuit supplied with the rectified network voltage for producing a DC voltage of a certain nominal value on an output node. The converter further includes a DC-DC converter supplied on an input node thereof with the DC voltage of the nominal value for producing a regulated DC voltage on an output node thereof. The DC-DC converter may use a clock whose frequency is selected between at least one low and one high value by a selection signal. A stand-by circuit may also be included for producing the selection signal based upon the current delivered to the load.
One advantageous feature of the converter of the invention is that it may include a control circuit having a comparator for receiving the selection signal and generating a disabling signal for the power factor correction pre-regulating circuit. The disabling signal is generated so long as the selection signal assumes a value corresponding to the low frequency value of the driving clock.
The power factor correction pre-regulating circuit may include a correction circuit that receives at an input thereof a signal representing the desired nominal DC voltage and produces a correction signal, and a power device driver that is supplied with the rectified network voltage and receives as inputs an enabling signal and the correction signal. The power device driver produces the nominal DC voltage on an output node thereof so long as the enabling signal is disabled. The power factor correction pre-regulating circuit may also include an enabling circuit for producing the enabling signal when the voltage on a control node is zero. Moreover, the power factor correction may be turned on or off by electrically isolating or coupling the control node to a reference voltage using a switch driven by the disabling signal.